360 



CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



Fig. 84. 



jar exposed to an atmosphere of 

 carbonic - acid gas, that gas will 

 force its way through the rubber 

 and mingle with the atmospheric 

 air in the jar, the rubber will be 

 pressed outward, as at b, and even- 

 tually may be burst. I found 

 that carbonic acid would thus 

 force its way against a pressure of 

 ten pounds on the square inch. If 

 the conditions be reversed, the jar being filled with car- 

 bonic acid and then exposed to the atmosphere, the In- 

 dia-rubber will be depressed as at a, and stretch so as 

 almost to sink to the bottom. Dr. Mitchell, of Philadel- 

 phia, has shown that this percolation will take place 

 though resisted by many inches of mercury. 



The same holds good as respects liquids. Thus water 

 will readily pass through animal membrane to mix with 

 alcohol against a pressure of fifteen pounds on the square 

 inch. 



In making experiments for determining the effect of 

 such pressures it is to be borne in mind that there are 

 certain disturbing circumstances which may vitiate the 

 results. Among these is that general leakage which 

 happens through the open pores of all tissues. Thus in 

 the experiment first referred to (Fig. 78) it might be 

 supposed that the force with which water passes through 

 animal membrane into alcohol is not greater than one 

 atmosphere, whereas in truth it is much more; but as 

 soon as the pressure within the vessel had amounted to 

 about one atmosphere, the alcohol escaped from the ves- 

 sel by general leakage from the whole surface of the 

 membrane as rapidly as the water entered. It is obvi- 

 ous that in a pore of sensible size those parts alone of a 

 passing liquid in contact with its substance are subjected 



